1. Technical Field
The present invention relates to a zoom lens system, an imaging device and a camera, and more particularly, to a small-size and high-image-quality zoon lens system suitable for digital still cameras, digital video cameras and the like, an imaging device having the zoom lens system, and the camera having the imaging device.
2. Description of the Background Art
In digital still cameras using a solid-state image sensor such as a CCD (charge coupled device) or a CMOS (complementary metal-oxide semiconductor), since a member such as an optical low-pass filter is disposed between the rearmost part of the lens elements and the solid-state image sensor, a lens system having a comparatively long back focal length is required. Moreover, the taking optical system of digital still cameras is required of an excellent telecentric characteristic in order to avoid shading which is a factor that decreases the quantity of peripheral light on the image surface.
Many forms of digital still cameras can be considered, and one form is a compact type. As the zoom lens system, one that is higher in magnification and imaging performance has been required. As zoom lens system suitable for the compact type, there have been proposed many three-unit zoom lens systems comprising from the object side a first lens unit having negative optical power, a second lens unit having positive optical power and a third lens unit having positive optical power. In the case of a magnification variation ratio of approximately 3×, these three-unit zoom lens systems can be formed so as to be compact and wide in the angle of view at the wide-angle limit. However, since the F-number at the telephoto limit is higher than that at the wide-angle limit, these three-unit zoom lens systems cannot be formed so as to be high in magnification.
Therefore, as zoom lens systems with which a comparatively high magnification is obtained without the F-number at the telephoto limit being much higher than that at the wide-angle limit, there have been proposed many four-unit zoom lens systems comprising from the object side a first lens unit having a positive refractive index, a second lens unit having a negative refractive index, and a third lens unit and a fourth lens unit each having a positive refractive index, for example, as described in Japanese Laid-Open Patent Publications Nos. 2001-13411, 2001-42215, 2002-72087 and 2004-212616.
However, in the zoom lens systems described in Japanese Laid-Open Patent Publication No. 2001-13411, although a magnification variation ratio of approximately 6× is obtained, since the number of lens elements is large, it is difficult to make the zoom lens systems compact when not in use. In the zoom lens systems described in Japanese Laid-Open Patent Publication No. 2001-42215, although the number of lens elements is small and it is therefore possible to make the zoom lens systems compact when not in use, the magnification variation ratio is as low as approximately 3×. In the zoom lens systems described in Japanese Laid-Open Patent Publication No. 2002-72087, the magnification variation ratio is approximately 3×, and in the zoom lens systems described in Japanese Laid-Open Patent Publication No. 2004-212616, the magnification ratio is approximately 4×.